This invention relates to a protective time-overcurrent induction relay having extremely inverse time-overcurrent response characteristics, and more particularly, to an electromagnet for use therein.
In an electrical power transmission system, a general requirement is that the system provide electrical energy continuously. Unfortunately, however, from time to time fault conditions arise which if left unchecked would cause damage to not only system sections initially involved, but also to associated sections. Accordingly, it is common for various sections within a power transmission system to be interconnected by means of circuit breakers which may be activated in the event of a disturbance to disengage from the system sections undergoing fault.
The conventional apparatus used in the above described scheme for monitoring electrical parameters and for activating circuit breakers in the event fault conditions warrant is the protective relay. Typically, such a relay is arranged within the system to sense a circuit parameter, as for example current flow, and to initiate a command signal for the activation of appropriate circuit breakers when and if the monitored circuit parameter exceeds a predetermined condition.
A relay as described above which responds to excess current is known in the art as an overcurrent relay. Further, a particular variety of overcurrent relay is the so called induction type which, owing to its principles of operation, as will be more fully explained below, is used exclusively in alternating current applications.
In an overcurrent induction relay, the a.c. current to be monitored is used to energize an electromagnet, which in turn is employed to drive an armature which is adapted to actuate a set of contacts. If a sufficient current condition is presented to the electromagnet, as for example, a sensed sustained overcurrent, due perhaps to a short circuit, the electromagnet is energized so as to rotate the armature sufficiently to effectively actuate the contact set, whereby a command signal may be communicated to an appropriate circuit breaker.
In certain applications such as subtransmission lines and feeder circuits, overcurrent relaying is extensively used because of its low cost and simplicity. In these applications, the relay is often required not only to operate rapidly in response to fault conditions that give rise to currents well above maximum load current, but also not to operate in response to currents that exceed maximum load current for short intervals during normal circuit operation. In this regard, it is to be noted that current may exceed maximum load value for a short time during a normal reclosure sequence as the system is required to pick up cold loads following a previous interruption.
In such applications, it is therefore desirable for the relay operating time to be longer for lower values of excess current so that overcurrents occasioned during normal operation will not cause the relay to initiate an unnecessary circuit breaker command. Where excess overcurrent is high, as for example during a short circuit, it is desirable for the relay operating time to be short in order to limit circuit damage. Further, it is to be noted that time-overcurrent relays having operating times which vary inversely with respect to energizing current magnitude, can be divided into three classes. Included in the first class is the so called "inverse" time-overcurrent relay. Included in the second class is the so called "very-inverse" time-overcurrent relay which exhibit a shorter operating time for a given normalized energization level than relays of the first class. Included in the third class is the so called "extremely inverse" time-overcurrent relay, which exhibits an even shorter operating time than relays of the second class for the same normalized energization level. Further, standards for the performance of such relays have been defined and established in graphs of operating time versus activating quantity published by such groups as the IEEE. A particular example of this is the I.E.E.E. Standard for Relays and Relay Systems Associated with Electric Power Apparatus IEEE Std. 313 - 1971 (ANSI C37.90 - 1971).
A widely-accepted prior design of a time-overcurrent introduction relay electromagnet for providing an extremely-inverse performance characteristic includes a magnetic core structure having three magnetic poles and three associated pole faces arranged to define an air gap therebetween through which a rotatable armature may pass. In this prior design, the electrical circuit for the electromagnet includes a transformer having a primary winding and a secondary winding coupled to the magnetic core two separate inductors also coupled to the core, a fixed value capacitor, and a variable resistor.
By such an arrangement, a magnetic flux distribution in response to monitored circuit current can be established in the air gap between the magnetic pole faces to interact with the rotatable armature therein. As a result, when the electromagnet is energized, a torque is applied to the rotatable armature which is a function of monitored current. Further, by adjusting the values of capacitance, inductance and resistance in the circuit of the electromagnet, the functional relationship between operating time and magnitude of activating current for the relay, falls within the defined limits for extremely-inverse performance as established by the aforesaid I.E.E.E. Standard 313-1971.
Since the above described design requires a transformer, several individual inductors, a fixed capacitance and a variable resistance, it tends to be costly and complex to manufacture, large and bulky in size and subject to reduced reliability by virtue of the numerous components required.